说明：  最小生成树问题 问题描述：若要在ｎ个城市之间架设通讯网络，只需要架设ｎ－１条线路即可。如何以最低的经济代价建设这个通讯网，是一个网的最小生成树问题。 设计要求：利用Prims算法求网的最小生成树；以文本形式输出生成树中各条边以及它们的权值。
(The issue of minimum spanning tree problem Description: n To set up a communication network between cities, only need to set up n-1 lines can be. How to minimize the economic costs of building the communications network is a network of minimum spanning tree problem. Design requirements: the use of algorithms for network Prims minimum spanning tree to the text of the various forms of output spanning tree edges and their weights.)